1. Field of the Invention
The present invention relates to an improved process for cryogenic separation of hydrocarbons in the presence of a relatively high carbon dioxide content wherein the hydrocarbon/carbon dioxide feed gas after precooling and turboexpansion is directed to a demethanizer column for the purpose of separating a methane/carbon dioxide overhead from an ethane-containing liquid hydrocarbon bottoms. More specifically, it is concerned with improving and stabilizing the operation of the demethanizer column by intentionally injecting a dehydrated methane-containing stripping gas to the bottom of the demethanizer.
2. Description of the Prior Art
High carbon dioxide content feed gases such as may be encountered in certain fields of the United States which may range in carbon dioxide content between 0.1 and 15% by volume continue to be a problem to the extent that cryogenic handling procedures may cause separation from the feed gas of carbon dioxide as "ice" with destructive effect on equipment and efficiency. Traditionally, carbon dioxide has been separated in advance of processing for hydrocarbon product separation with an added step such as amine treating, molecular sieve adsorption, methanol absorption or caustic treating or not removed, see for example U.S. Pat. No. 3,292,380 to Bucklin. In cases where the carbon dioxide is not removed from the feed gas, it must be removed from the product ethane by one of the above-mentioned sweetening processes.
In U.S. Pat. No. 3,595,782 to Bucklin et al., a method for producing an ethane-containing liquid hydrocarbon essentially free of both methane and carbon dioxide without carbon dioxide icing to foul equipment is disclosed. The process involves passing an expanded precooled feed gas through a demethanizer separation column at a temperature below -80.degree. F. and pressure consistent with maintaining the non-gaseous carbon dioxide dissolved in an ethane-containing liquid hydrocarbon phase. Thus, carbon dioxide icing does not occur and an ethane output substantially free of carbon dioxide; e.g., less than 0.1% carbon dioxide content, can be achieved. However, in operating such a plant on a commercial scale, the carbon dioxide content in the product is extremely sensitive to the reboiler temperature at the bottom of the demethanizer. For example, in one commercial scale plant, it was observed that at design level a one-degree change in the reboiler temperature would result in rapid temperature drops in selected tray temperatures and an increase in carbon dioxide content in the product from essentially zero to a value that exceeded the specification limit. Under such circumstances, closed-loop control of the reboiler temperature based on gas chromatograph carbon dioxide to ethane ratio measurements and the like is not possible.